Calibration method and calibration device

ABSTRACT

A calibration method for calibrating an attitude of a camera mounted on a vehicle using markers each arranged vertically and each positioned at a pre-designated height from a road surface is provided. The method includes: a first process including shooting an image of the markers with the camera, thereby generating a two-dimensional image; a second process including converting the two-dimensional image into a bird&#39;s eye view image on a specific plane so that the bird&#39;s eye view image reflects the height of each of the markers; and a third process including calculating a parameter of the camera based on a position difference between the markers in the specific plane obtained in the second process.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure is a U.S. national stage application ofInternational Patent Application No. PCT/JP2013/005276 filed on Sep. 5,2013 and is based on Japanese Patent Application No. 2012-220622 filedon Oct. 2, 2012, the contents of which are incorporated herein byreference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a technique for performing measurementconcerning mounting of a camera on a vehicle.

BACKGROUND

Known techniques for performing measurement (so-called calibration)concerning mounting of a camera on a vehicle include the following.

For example, in the method disclosed in the patent literature 1 listedbelow, a plurality of calibration patterns (targets) are placed on theroad surface in a predetermined positional relationship, a vehicle isplaced near the calibration patterns, and measurement is performed.

In the method disclosed in the patent literature 2 listed below,measurement is performed with a vehicle and a target device which arearranged in a fixed positional relationship using positioning means suchas joints and a scope.

PATENT LITERATURE

Patent Literature 1: JP 2009-288152A

Patent Literature 2: JP 2001-285681A

SUMMARY

In the method disclosed in the patent literature 1, to achieve highmeasurement accuracy, it is necessary to appropriately set the distancesfrom cameras to targets and the angles formed by the cameras withrespect to the targets. This requires the targets to be arranged over awide area on the road surface. As a result, a measurement space (flatspace for placing the targets used for measurement) several times aslarge as the space occupied by the vehicle is required.

In the case of the technique disclosed in the patent literature 2, atarget having a height from the road surface is used, so that the spaceused for measurement can be saved. To perform measurement, however,information about the positional relationship between the vehicle andthe target is required. It is, therefore, necessary to physically fixthe vehicle and the target in a predetermined positional relationshipusing joints or to adjust the position of the target using a scope meansso as to bring the vehicle and the target into a predeterminedpositional relationship. This makes measurement cumbersome.

The present disclosure has been made in view of the above problem, andit is an object of the present disclosure to provide a technique whichmakes measurement easy while saving the space required for measurement.

According to a first example of the present disclosure, a calibrationmethod is for calibrating an attitude of a camera mounted on a vehicleusing a plurality of markers each arranged vertically and eachpositioned at a pre-designated height from a road surface. Thecalibration method comprises: a first process including shooting animage of the plurality of markers with the camera, thereby generating atwo-dimensional image; a second process including converting thetwo-dimensional image, which is generated in the first process andrepresents the plurality of markers, into a bird's eye view image on aspecific plane so that the bird's eye view image reflects the height ofeach of the plurality of markers, wherein the specific plane is a roadsurface or on a plane parallel to the road surface; and a third processincluding calculating a parameter of the camera based on a positiondifference between the plurality of markers in the specific planeobtained in the second process.

According to the calibration method including the above processes, themarkers (targets) are positioned to have a height above the roadsurface. This can save the space required for measurement compared withcases in which markers are placed on the road surface. Also according tothe calibration method, it is necessary neither to physically fix, usingjoints, the vehicle and markers to keep them in a predeterminedpositional relationship nor to adjust the marker positions using a scopemeans so as to bring the markers into a predetermined positionalrelationship with the vehicle. Thus, when the calibration method isused, the measurement work to be performed is simpler than in caseswhere a prior-art technique is used.

According to a second example of the present disclosure, a calibrationmethod is for calibrating an attitude of a camera mounted on a vehicleusing a plurality of markers horizontally spaced apart in apre-designated positional relationship, each marker positioned at apre-designated distance from a road surface. The calibration methodcomprises: a first process including shooting an image of the pluralityof markers, thereby generating a two-dimensional image; a second processincluding converting the two-dimensional image, which is generated inthe first process and represents the plurality of markers, into a bird'seye view image on a specific plane so that the bird's eye view imagereflects the height of each of the plurality of markers, wherein thespecific plane is a road surface or a plane parallel to the roadsurface; and a third process including calculating a parameter of thecamera based on a difference between a distance between the plurality ofmarkers represented in the bird's eye view image on the specific planegenerated in the second process and a distance between the plurality ofmarkers determined based on the pre-designated positional relationship.

The above calibration device can also achieve effects similar to thoseachieved by the calibration method of the first example.

According to a third example of the present disclosure, a calibrationdevice is for calibrating an attitude of a camera mounted on a vehicleusing a plurality of markers each arranged vertically and eachpositioned at a pre-designated height from a road surface. Thecalibration device comprises: an image acquisition unit that acquires atwo-dimensional image representing the plurality of markers shot by thecamera; a conversion unit that converts the two-dimensional imageacquired by the image acquisition unit and representing the plurality ofmarkers into a bird's eye view image on a specific plane so that thebird's eye view image reflects the height of each of the plurality ofmarkers, wherein the specific plane is the road surface or on a planeparallel to the road surface; and a calculation unit that calculates aparameter of the camera based on a position difference between theplurality of markers represented in the specific plane acquired by theconversion unit.

According to the calibration method including the above processes, themarkers (targets) are positioned to have a height above the roadsurface. This can save the space required for measurement compared withcases in which markers are placed on the road surface. Also according tothe calibration method, it is necessary neither to physically fix, usingjoints, the vehicle and markers to keep them in a predeterminedpositional relationship nor to adjust the marker positions using a scopemeans so as to bring the markers into a predetermined positionalrelationship with the vehicle. Thus, when the calibration method isused, the measurement work to be performed is simpler than in caseswhere a prior-art technique is used.

According to a fourth example of the present disclosure, a calibrationdevice is for calibrating an attitude of a camera mounted on a vehicleusing a plurality of markers horizontally spaced apart in apre-designated positional relationship, each marker having apre-designated distance from a road surface. The calibration devicecomprises: an image acquisition unit that acquires a two-dimensionalimage representing the plurality of markers shot by the camera; aconversion unit that converts the two-dimensional image acquired by theimage acquisition unit and representing the plurality of markers into abird's eye view image on a specific plane so that the bird's eye viewimage reflects the height of each of the plurality of markers, whereinthe specific plane is a road surface or on a plane parallel to the roadsurface; and a calculation unit that calculates a parameter of thecamera based on a difference between a distance between the plurality ofmarkers represented in the bird's eye view image on the specific planeacquired by the conversion unit and a distance between the plurality ofmarkers determined based on the pre-designated positional relationship.

The above calibration device can also achieve effects similar to thoseachieved by the calibration method of the third example.

BRIEF DESCRIPTION OF DRAWINGS

The above and other objects, features and advantages of the presentdisclosure will become more apparent from the following detaileddescription made with reference to the accompanying drawings. In theaccompanying drawings:

FIG. 1 is a block diagram schematically showing a configuration of animage processing system according to embodiments;

FIG. 2A is an explanatory diagram showing camera positions and apositional relationship between a vehicle and markers according to afirst embodiment;

FIG. 2B is an explanatory diagram showing camera positions and apositional relationship between a vehicle and markers according to asecond embodiment;

FIG. 3 is a flowchart of attitude parameter determination processing 1;

FIG. 4A is a diagram showing an example of a camera-shot image;

FIG. 4B is a diagram illustrating calculation of a distance betweenmarkers represented in a bird's eye view image converted from acamera-shot image;

FIG. 4C is a diagram for explaining an actual distance between markers;

FIG. 4D is a diagram illustrating a manner in which a line segmentdefined based on measurement-based marker positions is translated androtated for alignment with a line segment defined based on actual markerpositions;

FIG. 4E is a diagram showing the line segments aligned by translationand rotation;

FIG. 5A is a diagram conceptually showing camera positions determined bymeasurement (marker coordinate system);

FIG. 5B is a diagram conceptually showing a coordinate system based on avehicle including ideally positioned cameras;

FIG. 5C is a diagram illustrating a manner in which a line segmentdefined based on measurement-based camera positions is translated androtated for alignment with a line segment defined based on ideal camerapositions;

FIG. 5D is a diagram conceptually showing results of conversion of thehorizontal components of the position of each camera and the yaw angleof the shooting direction of each camera from a marker coordinate systemto a vehicle coordinate system;

FIG. 6 is a flowchart of attitude parameter determination processing 2;

FIG. 7A is a diagram showing an example of a camera-shot image;

FIG. 7B is a diagram illustrating calculation of a position differencebetween markers on each pole represented in a bird's eye view imageconverted from a camera-shot image;

FIG. 7C is a diagram which illustrates synthesizing bird's eye viewimages into one using a camera coordinate system of one of the bird'seye view images and identifying the horizontal components of theposition of each camera and the yaw angle of the shooting direction ofeach camera;

FIG. 8A is a diagram conceptually showing the position of each cameradetermined by measurement (camera coordinate system);

FIG. 8B is a diagram conceptually showing a coordinate system based on avehicle including ideally positioned cameras;

FIG. 8C is a diagram illustrating a manner in which a line segmentdefined based on measurement-based camera positions is translated androtated for alignment with a line segment defined based on ideal camerapositions; and

FIG. 8D is a diagram conceptually showing results of conversion of thehorizontal components of the position of each camera and the yaw angleof the shooting direction of each camera from a camera coordinate systemto a vehicle coordinate system.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described with referenceto drawings. The embodiments being described below are, however, merelyillustrative and not at all restrictive of other alternative embodimentsof the present disclosure. The embodiments of the present disclosure areinclusive of embodiments configured by omitting a part of theconfiguration of any of the following embodiments as long as objects ofthe present disclosure are achievable. The embodiments of the presentdisclosure are also inclusive of embodiments configured by appropriatelycombining the following embodiments.

<Description of Configuration>

As shown in FIG. 1, an image processing system 5 of embodiments includescameras 11 a to 11 d, an image processing device 21, and a displaydevice 31.

The cameras 11 a to 11 d are each a wide-angle camera including an imagesensor such as a CCD or a CMOS. They each shoot an area around a vehicleand output an image generated by shooting to the image processing device21 at a predetermined frequency (e.g., at a frequency of 60 frames persecond). In the present embodiment, as shown in FIG. 2A, the camera 11 ais mounted in a front-end center portion of the vehicle so as to shoot aforward peripheral area of the vehicle, the camera 11 b is mounted on aright-side surface of the vehicle (specifically, on the right doormirror) so as to shoot a right-side peripheral area of the vehicle, thecamera 11 c is mounted in a rear-end center portion of the vehicle so asto shoot a rearward peripheral area of the vehicle, and the camera 11 dis mounted on a left-side surface of the vehicle (specifically, on theleft door mirror) so as to shoot a left-side peripheral area of thevehicle. In the following description, when the cameras need not bedistinguished from one another, each of them may be commonly referred tosimply as a “camera 11.”

Reverting to FIG. 1, the image processing device 21 includes imagestorage units 22 a to 22 d, an operation unit 23, an attitude parameterstorage unit 24, and a control unit 25.

The image storage units 22 a to 22 d include storage devices such asDRAMs and store the images shot by and sequentially outputted from thecameras 11, the images stored in each of the image storage units 22 a to22 d totaling in time to a predetermined amount of time (e.g., the lastten seconds). In doing this, the image storage units 22 a to 22 d storethe images outputted from the cameras 11 a to 11 d, respectively. In thefollowing description, when the image storage units need not bedistinguished from one another, each of them may be commonly referred tosimply as an “image storage unit 22.”

The operation unit 23 includes a touch panel provided on a displayscreen of the display device 31 and mechanical key switches provided,for example, around the display device 31. The operation unit 23 allows,for example, a driver of the vehicle to input various operatinginstructions from the operation unit 23.

The attitude parameter storage unit 24 includes a non-volatile storagedevice, for example, a flash memory. The attitude parameter storage unit24 stores, for example, attitude parameter values of each camera 11(horizontal and vertical components of the position of each camera 11and also the pitch angle, roll angle and yaw angle of the shootingdirection of each camera 11), and programs for execution by the controlunit 25. The images taken by each camera 11 are processed (e.g., forconversion into bird's eye view images) using the corresponding attitudeparameters stored in the attitude parameter storage unit 24. Theattitude parameters are also used in determining if there is any camera11 the mounted position or angle of which has become abnormal, forexample, due to vibration of the vehicle making it necessary to issue awarning.

The control unit 25 includes a microcomputer including a CPU, RAM, ROMand I/O device and performs various processing operations by readingprograms stored in the attitude parameter storage unit 24.

The display device 31 includes, for example, a liquid crystal display oran organic EL display and can display images shot by the cameras 11 andprocessed by the image processing device 21.

Description of Operation of First Embodiment

Next, the operation of an image processing device 21 of a firstembodiment will be described. In the following, description will centeron the processing related with the present disclosure, and descriptionwill be omitted as to the known processing performed to display imagestaken by vehicle-mounted cameras of the type being discussed herein(e.g., processing performed to convert images taken by cameras intobirds' eye view images and synthesize and display forecasted tracks ofwheels on a display device, for example, to support backing of a vehicleinto a garage).

In the first embodiment, to calibrate cameras mounted on a vehicle, thevehicle is placed in an area surrounded by four marker poles 41 a to 41d as shown in FIG. 2A. The four marker poles 41 a to 41 d are columnarpoles fixed to axially vertically extend from a road surface. The markerpoles 41 a to 41 d include markers 42 a to 42 d, respectively. Themarkers 42 a to 42 d are positioned in longitudinally middle portions ofthe marker poles 41 a to 41 d, respectively, to face, each at a sameheight from the road surface, in the directions toward the vehicle.Namely, the markers 42 a to 42 d are positioned in a plane parallel tothe road surface. The positions in the plane of the markers 42 a to 42 dare pre-designated. Therefore, the actual positions of the markers 42 ato 42 d are identified by coordinates in a coordinate system (markercoordinate system) based on a predetermined position. The markers 42 ato 42 d may be any objects as long as they can be shot by the cameras 11mounted on the vehicle and their images shot by the cameras 11 can berecognized by the image processing device 21. For example, markersdiffering in color from the marker poles 41 a to 41 or light emittingdevices such as LEDs are appropriate for use as the maker poles 41 a to41 d.

In the following description, when the marker poles need not bedistinguished from one another, each of them may be commonly referred tosimply as a “marker pole 41.” Also, when the markers need not bedistinguished from one another, each of them may be commonly referred tosimply as a “marker 42.”

Next, with reference to the flowchart shown in FIG. 3, attitudeparameter determination processing 1 performed for camera calibration bythe image processing device 21 will be described. When personnel incharge of measurement inputs, from the operation unit 23, an instructionfor updating the attitude parameters, the attitude parameterdetermination processing 1 is read into the program control unit 25 fromthe attitude parameter storage unit 24 and starts being performed. Suchan instruction for updating the attitude parameters is issued only afterthe vehicle is placed as shown in FIG. 2A enabling each camera 11 toshoot two of the marker poles 41 (two of the markers 42). The controlunit 25 performs S105 to S140, being described in the following,separately for each of the cameras 11 a to 11 d, then, after completionof S105 to S140 for each camera, performs S145 and S150 once for allcameras.

Upon starting the attitude parameter determination processing 1, thecontrol unit 25 obtains the latest camera-shot image from a selected oneof the image storage units 22 (S105). Namely, the control unit 25 readsimage data from one of the image storage units 22 a to 22 d and lays itout in a RAM, not shown, included in the control unit 25.

The control unit 25 then detects, from the camera-shot image obtained,the coordinates of the markers 42 (5110). Namely, as shown in FIG. 4A,the control unit 25 identifies, in an image coordinate system, thecenters of the markers 42 a and 42 b shown in the camera-shot image. Theposition of each marker 42 may be identified, for example, bydetermining a spot representing a large color or luminance difference onthe corresponding image or, in cases where each marker 42 includes alight emitting device, by making the light emitting device flash andmeasuring the difference between an image taken before flashing of thelight emitting device and an image taken after flashing of the lightemitting device.

Reverting to FIG. 3, the control unit 25 subsequently converts thecamera-shot image into a bird's eye view image taking into considerationthe heights of the markers 42 (S115). The method of bird's eye viewconversion is well known, so that it will not be described herein. Themethod is described in detail in JP H10-211849A. When performing S115for the first time after starting execution of the attitude parameterdetermination processing 1 (when converting a camera-shot image into abird's eye view image), the attitude parameter values of each camera 11to be used may be the initial parameter values provided, for example,when the vehicle is shipped from the factory or may be the parametervalues stored in the attitude parameter storage unit 24 (the parametervalues stored when the attitude parameter determination processing 1 wasexecuted last time). When a camera-shot image is converted into a bird'seye view image, the entire camera-shot image may be converted or only apart of the camera-shot image (e.g., only a portion showing the markers42) may be converted. It is possible to convert an image of the markers42 into a bird's eye view image on a horizontal plane of the same heightas the markers 42, convert an image of other objects than the markers 42into a bird's eye view image on the road surface, and synthesize thebird's eye view images into one.

Subsequently, the control unit 25 calculates distance D between themarkers shown in the bird's eye view image thus generated (S120).Namely, as shown in FIG. 4B, distance D between the markers 42 a and 42b is calculated based on the coordinates of the marker 42 a and thecoordinates of the marker 42 b in the coordinate system for the targetcamera 11 (camera coordinate system).

Reverting to FIG. 3, the control unit 25 determines whether or not thedistance D between the markers calculated in S120 can be evaluated to beclose enough to a prescribed value (S125). The “prescribed value”represents the actual distance between the markers, i.e. the distancebetween the two predetermined markers 42 calculated based on theircoordinates in the marker coordinate system (see FIG. 4C). The distanceD may be evaluated to be “close enough” to a prescribed value, forexample, when the ratio of the difference between the distance D betweenthe markers calculated in S120 to the prescribed value is equal to orsmaller than a prescribed ratio.

When the control unit 25 determines in S125 that the distance D betweenthe markers calculated in S120 can be evaluated to be close enough tothe prescribed value, the control unit 25 advances processing to S135.On the other hand, when the control unit 25 determines that the distanceD calculated in S120 cannot be evaluated to be close enough to theprescribed value, the control unit 25 advances processing to S130.

In S130 entered after the distance D between the markers calculated inS120 is evaluated to be not close enough to the prescribed value, thecontrol unit 25 selects candidate values to be the next values of thevertical component (z) of the position of the camera 11 and the pitchangle and roll angle of the shooting direction of the camera 11. Theseparameters are among the attitude parameters used to generate a bird'seye view image in S115. The candidate values for next use are a set ofvalues of the above elements with at least one of them updated by asmall amount (e.g., by an amount equivalent to 0.1% of the amount beforethe update).

After selecting the candidate values for next use in S130, the controlunit 25 returns processing to S115, described above, and again convertsthe camera-shot image into a bird's eye view image using the attitudeparameter values selected as described above.

On the other hand, when processing is advanced to S135 after thedistance D between the markers calculated in S120 is evaluated to beclose enough to the prescribed value, the control unit 25 stores thevertical component (z) of the position of the camera 11 and the pitchangle and roll angle of the shooting direction of the camera 11 that areused to generate a bird's eye view image in S115 last time in theattitude parameter storage unit 24.

Subsequently, the control unit 25 moves and rotates the bird's eye viewimage (image represented in the camera coordinate system) obtained bybird's eye view conversion last performed in S115 until the line segmentconnecting the markers 42 shown in the bird's eye view image is alignedwith the line segment connecting the markers 42 in their actualpositions (actual marker positions represented in the marker coordinatesystem). At the same time, the control unit 25 also moves and rotatesthe origin and coordinate axes of the camera coordinate system in thebird's eye view image obtained in S115. Based on the origin andcoordinate axes of the moved and rotated camera coordinate system, thecontrol unit 25 calculates, in the marker coordinate system, thehorizontal components (x, y) of the position of the camera 11 and theyaw angle of the shooting direction of the camera 11 (S140). In otherwords, in S140, the camera position (x, y) and yaw angle are identifiedwith the line segment connecting the markers moved and rotated inaccordance with the prescribed marker arrangement. When moving a linesegment for alignment with another line segment, the line segment may bemoved so as to align the midpoints of the two line segments, but it mayalso be moved in an alternative manner.

The processing performed in S140 will be described more specificallywith reference to FIGS. 4B to 4E.

FIG. 4B shows an image selectively showing, based on a bird's eye viewimage converted from a camera-shot image, the markers 42 a and 42 b andpoles 41 a and 41 b and, in the image, a line segment 44 connecting themarkers 42 a and 42 b is defined. The position of the camera 11 used toshoot the image corresponds to the origin Oc of the camera coordinatesystem and the shooting direction of the camera exists on a Yc-Zc plane.

FIG. 4C shows the markers 42 a and 42 b in actual arrangement (actualmarker positions). In FIG. 4C, a line segment 45 connecting the markers42 a and 42 b is defined. The origin Om of the coordinate system for themarkers 42 (marker coordinate system) is shown in FIG. 4C, but thelocation of the origin Om is not limited to where it is shown in FIG.4C.

The manner in which the line segment 44 shown in FIG. 4B is translatedand rotated for alignment with the line segment 45 shown in FIG. 4C isillustrated in FIG. 4D. When the line segment 44 is translated androtated as shown in FIG. 4D, the origin Oc and coordinate axes of thecamera coordinate system are also moved (the manner in which they aremoved is not shown).

FIG. 4E shows the line segment 44 shown in FIG. 4B after beingtranslated and rotated for alignment with the line segment 45 shown inFIG. 4C. In FIG. 4E, the position of the origin Oc and coordinate axesof the camera coordinate system represented in the marker coordinatesystem can be checked. The horizontal components (x and y components) ofthe coordinate position of the origin Oc shown in FIG. 4E represent thehorizontal components of the position of the camera 11 in the markercoordinate system. The angle formed between the Yc-Zc plane of thecamera coordinate system and the Ym-Zm plane represents the yaw angle ofthe shooting direction of the camera 11 represented in the markercoordinate system.

Reverting to FIG. 3, the control unit 25 subsequently converts thehorizontal components of the position of each camera 11 and the yawangle of the shooting direction of each camera 11 calculated in S140based on the marker coordinate system into the horizontal components ofthe position of each camera 11 and the yaw angle of the shootingdirection of each camera 11 based on the vehicle coordinate system(S145). This conversion processing will be described with reference toFIGS. 5A to 5D.

FIG. 5A conceptually shows the positions of the cameras 11 a to 11 d inthe marker coordinate system based on the results of performing S105through S140 for each of the cameras 11 a to 11 d. In the example shownin FIG. 5A, the camera 11 a mounted in a front-end center portion of thevehicle and the camera 11 c mounted in a rear-end center portion of thevehicle are selected and a line segment 51 connecting the positions ofthe two cameras is defined. The cameras to be selected are not limitedto the above two. A different combination of cameras may be selected.

FIG. 5B conceptually shows a coordinate system (vehicle coordinatesystem) based on a front-end center portion of a vehicle includingcameras in an ideal arrangement. The vehicle coordinate system has anorigin at the position of the camera 11 a mounted in the front-endcenter portion of the vehicle, a Y axis horizontally extending forwardlyof the vehicle, an X axis horizontally extending rightwardly of thevehicle, and a Z axis upwardly (vertically) extending from the vehicle.In this example, too, the camera 11 a mounted in the front-end centerportion of the vehicle and the camera 11 c mounted in a rear-end centerportion of the vehicle are selected and a line segment 52 connecting thetwo cameras is defined.

With the above vehicle coordinate system provided, the line segment 51shown in FIG. 5A is rotated and translated until the line segment 51 isaligned with the line segment 52 defined in FIG. 5B. At this time, thehorizontal components of the position of each camera 11 and the yawangle of the shooting direction of each camera 11 are also rotated andtranslated correspondingly. This converts the horizontal components ofthe position of each camera 11 and the yaw angle of the shootingdirection of each camera 11 based on the marker coordinate system intothose based on the vehicle coordinate system.

FIG. 5C shows the manner in which the ling segment 51 shown in FIG. 5Ais rotated and translated to be aligned with the line segment 52 shownin FIG. 5B. It is conceivable to move the line segment 51 such that themidpoint of the line segment 51 is aligned with the midpoint of the linesegment 52, but the line segment 51 may be moved in an alternativemanner.

FIG. 5D conceptually shows the results of converting the horizontalcomponents of the position of each camera 11 and the yaw angle of theshooting direction of each camera 11 based on the marker coordinatesystem into those based on the vehicle coordinate system. Comparing FIG.5D and FIG. 5B, it is shown that each camera shown in FIG. 5D isoriented differently from its orientation shown in FIG. 5B. Thus, it isconfirmed that the position of each camera 11 shown in FIG. 5A can beidentified in the vehicle coordinate system.

Reverting to FIG. 3, the control unit 25 subsequently stores thehorizontal components of the position of each camera 11 and the yawangle of the shooting direction of each camera 11 based on the vehiclecoordinate system as determined by conversion in S145 in the attitudeparameter storage unit 24 (S150). The control unit 25 then ends theprocessing (attitude parameter determination processing 1).

Effects of First Embodiment

The operation of the image processing device 21 according to the firstembodiment has been described. According to the first embodiment, themarkers 42 are positioned to have a height above the road surface. Thiscan save the space required for measurement compared with cases in whichmarkers are placed on the road surface. Also according to the firstembodiment, it is necessary neither to physically fix, using joints, thevehicle and markers to keep them in a predetermined positionalrelationship nor to adjust the marker positions using a scope means soas to bring the markers into a predetermined positional relationshipwith the vehicle. Thus, the measurement work to be performed is simpleaccording to the first embodiment compared with cases in which aprior-art technique is used.

Furthermore, the image processing device 21 according to the firstembodiment can calculate the values of all parameters (positions, pitchangles, roll angles and yaw angles) of all cameras 11 mounted on thevehicle.

Description of Operation of Second Embodiment

Next, the operation of an image processing device 21 of a secondembodiment will be described. The following description of the secondembodiment will center on aspects of the second embodiment differentfrom those of the first embodiment, and descriptions of operationsidentical between the first and second embodiments will be omitted.

In the second embodiment, too, to calibrate cameras mounted on avehicle, the vehicle is placed in an area surrounded by four markerpoles 41 a to 41 d as shown in FIG. 2B. The four marker poles 41 a to 41d include markers 42 a to 42 d, respectively. The markers 42 a to 42 d(hereinafter also referred to as “lower markers”) are positioned inlongitudinally middle portions of the marker poles 41 a to 41 d,respectively, to face, at a same height from the road surface, in thedirections toward the vehicle. The four marker poles 41 a to 41 d alsoinclude markers 43 a to 43 d, respectively. The markers 43 a to 43 d(hereinafter also referred to as “upper markers”) are positioned inportions near the upper ends of the marker poles 41 a to 41 d,respectively, to face, at a same height from the road surface, in thedirections toward the vehicle. These markers 42 a to 42 d and 43 a to 43d are vertically aligned on the respective marker poles. Unlike in thefirst embodiment, the positions of the markers 42 a to 42 d and 43 a to43 d need not be pre-identified in a specific coordinate system, buttheir height from the road surface is predetermined.

In the following description, when the markers 42 a to 42 d need not bedistinguished from one another, each of them may be commonly referred tosimply as a “marker 42” (or as a “lower marker 42”). Similarly, when themarkers 43 a to 43 d need not be distinguished from one another, each ofthem may be commonly referred to simply as a “marker 43” (or as an“upper marker 43”).

Next, with reference to the flowchart shown in FIG. 6, attitudeparameter determination processing 2 performed by the image processingdevice 21 for camera calibration will be described. When personnel incharge of measurement inputs, from the operation unit 23, an instructionfor updating the attitude parameters, the attitude parameterdetermination processing 2 is read into the program control unit 25 fromthe attitude parameter storage unit 24 and starts being performed. Suchan instruction for updating the attitude parameters is issued only afterthe vehicle is placed as shown in FIG. 2B enabling each camera 11 toshoot the markers positioned on two of the marker poles 41. The controlunit 25 performs S205 to S235, being described in the following,separately for each of the cameras 11 a to 11 d, then, after completionof S205 to S235 for each camera, performs S240 to S250 once for allcameras.

Upon starting the attitude parameter determination processing 2, thecontrol unit 25 obtains the latest camera-shot image from a selected oneof the image storage units 22 (S205).

The control unit 25 then detects, from the camera-shot image obtained,the coordinates of the lower markers 42 and upper markers 43 (S210).Namely, as shown in FIG. 7A, the control unit 25 identifies, in an imagecoordinate system, the centers of the markers 42 a, 42 b, 43 a and 43 bshown in the camera-shot image.

Reverting to FIG. 6, the control unit 25 subsequently converts thecamera-shot image into a bird's eye view image (S215). This is done bytaking into consideration the heights of the lower markers 42 and uppermarkers 43.

Subsequently, the control unit 25 calculates the position difference, inthe bird's eye view image, between the lower marker 42 and the uppermarker 43 on each pole (S220). For example, as shown in FIG. 7B,distance d1 between the positions of the lower marker 42 a and uppermarker 43 a and distance d2 between the positions of the lower marker 42b and upper marker 43 b are calculated in a coordinate system (cameracoordinate system) based on the camera 11 a used to shoot the image.

Reverting to FIG. 6, the control unit 25 then determines whether or notthe distance between the positions of the lower marker 42 and uppermarker 43 on each pole can be evaluated as a minimum (S225). The“minimum” is ideally preferably 0, but it may be a value infinitelyproximate to 0 with an error taken into consideration.

When it is determined in S225 that the distance between the positions ofthe lower marker 42 and upper marker 43 on each pole measured in S220can be evaluated as a minimum, the control unit 25 advances processingto S235. When, on the other hand, it is determined in S225 that thedistance between the positions of the lower marker 42 and upper marker43 on each pole measured in S220 cannot be evaluated as a minimum, thecontrol unit 25 advances processing to S230.

In S230 entered when it is determined in S225 that the distance betweenthe positions of the lower marker 42 and upper marker 43 on each polemeasured in S220 cannot be evaluated as a minimum, the control unit 25selects candidate values to be the next values of the vertical component(z) of the position of the camera 11 and the pitch angle and roll angleof the shooting direction of the camera 11. These elements are among theattitude parameters used to generate a bird's eye view image in S215.The candidate values for next use are a set of values of the aboveelements with at least one of them updated by a small amount (e.g., byan amount equivalent to 0.1% of the amount before the update).

After selecting the candidate values for next use in S230, the controlunit 25 returns processing to S215 described above and again convertsthe camera-shot image into a bird's eye view image using the attitudeparameter values selected as described above.

In S235 entered when it is determined in S225 that the distance betweenthe positions of the lower marker 42 and upper marker 43 on each polemeasured in S220 can be evaluated as a minimum, the control unit 25stores the vertical component (z) of the position of the camera 11 andthe pitch angle and roll angle of the shooting direction of the camera11 that are used to generate a bird's eye view image in S215 last timein the attitude parameter storage unit 24.

The control unit 25 performs the above processing (S205 to S235) foreach of the cameras 11 a to 11 d and, after obtaining bird's eye viewimages (latest bird's eye view images generated by bird's eye viewconversion performed in S215) corresponding to all cameras 11, startsthe processing of S240.

The control unit 25 synthesizes the bird's eye view images (images inthe respective camera coordinate systems) corresponding to all cameras11 into one (S240). This is done by translating and rotating thedifferent images for alignment such that the same markers shown in thedifferent images are aligned, respectively. At this time, the originsand coordinate axes of the camera coordinate systems of the cameras 11except for one selected as a reference camera coordinate system are alsotranslated and rotated. In other words, in the camera coordinate systemof one of the cameras 11, the bird's eye view images corresponding tothe other cameras 11 are synthesized and the position (x, y) and yawangle of each camera are identified.

Image synthesis will be described with reference to FIG. 7C. Aforward-area bird's eye view image 61 a is a bird's eye view imagegenerated based on an image shot by the camera 11 a mounted in afront-end center portion of the vehicle; a right-area bird's eye viewimage 61 b is a bird's eye view image generated based on an image shotby the camera 11 b mounted in a right-side portion of the vehicle; arearward-area bird's eye view image 61 c is a bird's eye view imagegenerated based on an image shot by the camera 11 c mounted in arear-end center portion of the vehicle; and a left-area bird's eye viewimage 61 d is a bird's eye view image generated based on an image shotby the camera 11 d mounted in a left-side portion of the vehicle. In theexample shown in FIG. 7C, using the camera coordinate system of theforward-area bird's eye view image 61 a as a reference camera coordinatesystem, the different images are synthesized into one by translating androtating the other bird's eye view images for alignment such that thesame markers shown on the different images are aligned, respectively. Atthis time, the horizontal components of the position of each camera 11and the yaw angle of the shooting direction of each camera 11corresponding to each bird's eye view image are also translated androtated. This makes it possible to calculate the horizontal componentsof the shooting position of each camera 11 and the yaw angle of theshooting direction of each camera 11 based on the camera coordinatesystem of the forward-area bird's eye view image 61 a.

Reverting to FIG. 6, the control unit 25 subsequently converts thehorizontal components of the position of each camera 11 and the yawangle of the shooting direction of each camera 11 calculated in S240based on the camera coordinate system for each camera 11 into thehorizontal components of the position of each camera 11 and the yawangle of the shooting direction of each camera 11 in the vehiclecoordinate system (S245). This conversion will be described withreference to FIGS. 8A to 8D.

FIG. 8A conceptually shows the horizontal components of the position ofeach camera 11 and the yaw angle of the shooting direction of eachcamera 11 in the camera coordinate system. In the example shown in FIG.8A, the camera 11 a mounted in a front-end center portion of the vehicleand the camera 11 c mounted in a rear-end center portion of the vehicleare selected and a line segment 71 connecting the positions of the twocameras is defined. The cameras to be selected are not limited to theabove two. A different combination of cameras may be selected.

FIG. 8B conceptually shows a coordinate system (vehicle coordinatesystem) based on a front-end center portion of a vehicle includingcameras in an ideal arrangement. The vehicle coordinate system has anorigin at the position of the camera 11 a mounted in the front-endcenter portion of the vehicle, a Y axis horizontally extending forwardlyof the vehicle, an X axis horizontally extending rightwardly of thevehicle, and a Z axis upwardly (vertically) extending from the vehicle.In this example, too, the camera 11 a mounted in the front-end centerportion of the vehicle and the camera 11 c mounted in a rear-end centerportion of the vehicle are selected and a line segment 72 connecting thepositions of the two cameras is defined.

With the above vehicle coordinate system provided, the line segment 71shown in FIG. 8A is rotated and translated until the line segment 71 isaligned with the line segment 72 defined in FIG. 8B, causing thehorizontal components of the position of each camera 11 and the yawangle of the shooting direction of each camera 11 to be correspondinglyrotated and translated. By doing this, the horizontal components of theposition of each camera 11 and the yaw angle of the shooting directionof each camera 11 are converted into those based on the vehiclecoordinate system.

FIG. 8C shows the manner in which the line segment 71 shown in FIG. 8Ais rotated and translated to be aligned with the line segment 72 shownin FIG. 8B. It is conceivable to move the line segment 71 such that themidpoint of the line segment 71 is aligned with the midpoint of the linesegment 72, but the line segment 71 may be moved in an alternativemanner.

FIG. 8D conceptually shows the results of converting the horizontalcomponents of the position of each camera 11 and the yaw angle of theshooting direction of each camera 11 based on the camera coordinatesystem into those based on the vehicle coordinate system. Comparing FIG.8D and FIG. 8B, it is shown that each camera shown in FIG. 8D isoriented differently from its orientation shown in FIG. 8B. Thus, it isconfirmed that the position of each camera 11 shown in FIG. 8A can beidentified in the vehicle coordinate system.

Reverting to FIG. 6, the control unit 25 subsequently stores thehorizontal components of the position of each camera 11 and the yawangle of the shooting direction of each camera 11 based on the vehiclecoordinate system as determined by image conversion in S245 in theattitude parameter storage unit 24 (S250). The control unit 25 then endsthe processing (attitude parameter determination processing 2).

Effects of Second Embodiment

The operation of the image processing device 21 according to the secondembodiment has been described. According to the second embodiment, themarkers 42 and 43 are positioned to have a height above the roadsurface. This can save the space required for measurement compared withcases in which markers are placed on the road surface. Also according tothe second embodiment, it is necessary neither to physically fix, usingjoints, the vehicle and markers to keep them in a predeterminedpositional relationship nor to adjust the marker positions using a scopemeans so as to bring the markers into a predetermined positionalrelationship with the vehicle. Thus, the measurement work to beperformed is simple according to the second embodiment compared withcases in which a prior-art technique is used.

Furthermore, the image processing device 21 according to the secondembodiment can calculate the values of all parameters (positions, pitchangles, roll angles and yaw angles) of all cameras 11 mounted on thevehicle.

Also, according to the second embodiment compared with the firstembodiment, the positions of the marker poles 41 need not bepre-designated (need not be strictly in predetermined positions). It is,therefore, possible to set up the marker poles 41 before performingmeasurement without necessity to keep them set up in place. Thus,measurement can be performed more easily than in the first embodiment.

Other Embodiments

(1) Even though, in the above embodiments, all the attitude parametersof the cameras 11 are calculated, it is not necessarily required tocalculate all the attitude parameters. For example, an alternativeapproach may be used in which only the pitch angle and yaw angle of theshooting direction of each camera 11 are calculated.

(2) The values of the attitude parameters to be stored in the attitudeparameter storage unit 24 need not necessarily be those based on thecoordinate systems described in connection with the above embodiments.The attitude parameters converted into those based on an alternativecoordinate system may also be stored in the attitude parameter storageunit 24.

(3) Even though, in the above embodiments, the attitude parameterdetermination processing is performed entirely by the control unit 25 ofthe image processing device 21 mounted in the vehicle, the attitudeparameter determination processing may partly be performed by anotherdevice different from the image processing device 21. In such a case,the another device may be mounted in a location (outside the vehicle)different from where the image processing device 21 is mounted, therebyallowing the image processing device 21 to perform the attitudeparameter determination processing while communicating with the anotherdevice via a communication line.

In the above embodiments, S105 and S205 performed by the control unit 25correspond to an example of a first process and an image acquisitionunit (or means). S115 and S215 performed by the control unit 25correspond to an example of a second process and conversion (or means).S140, S145, S240 and S245 performed by the control unit 25 correspond toan example of a third process and a calculation unit (or means).

According to the present disclosure, various forms of calibrationmethods and calibration devices can be provided.

For example, a calibration method according to the first example of thepresent disclosure is for calibrating an attitude of a camera mounted ona vehicle using a plurality of markers each arranged vertically and eachpositioned at a pre-designated height from a road surface. Thecalibration method comprises: a first process including shooting animage of the plurality of markers with the camera, thereby generating atwo-dimensional image; a second process including converting thetwo-dimensional image, which is generated in the first process andrepresents the plurality of markers, into a bird's eye view image on aspecific plane so that the bird's eye view image reflects the height ofeach of the plurality of markers, wherein the specific plane is a roadsurface or on a plane parallel to the road surface; and a third processincluding calculating a parameter of the camera based on a positiondifference between the plurality of markers in the specific planeobtained in the second process.

According to the calibration method including the processes describedabove, the markers (targets) are positioned to have a height above theroad surface. This can save the space required for measurement comparedwith cases in which markers are placed on the road surface. Alsoaccording to the calibration method, it is necessary neither tophysically fix, using joints, the vehicle and markers to keep them in apredetermined positional relationship nor to adjust the marker positionsusing a scope means so as to bring the markers into a predeterminedpositional relationship with the vehicle. Thus, when the calibrationmethod is used, the measurement work to be performed is simpler than incases where a prior-art technique is used.

In the third process, the parameter includes at least one of a verticalcomponent of a position of the camera, a pitch angle of the camera, anda roll angle of the camera. Such a parameter can be accuratelycalculated in the third process.

The vehicle may be mounted with either a single camera or a plurality ofcameras. When cameras are mounted, the three processes may be performedas follows (third example). In the first process, the plurality ofmarkers are shot by each camera and the two-dimensional image isgenerated for each camera. In the second process, each of thetwo-dimensional images generated in the first process is converted intothe bird's eye view image. In the third process, at least either a yawangle of each camera or a horizontal component of the position of eachcamera is further calculated as the parameter of the each camera, sothat a particular marker shot by one camera have the same position inthe converted bird's eye view image as the particular marker shot byanother camera.

The calibration method including the above processes makes it possibleto calculate at least either the yaw angle of the position of each ofcameras mounted on a vehicle or the horizontal components of theposition of each camera in a same coordinate system, while saving thespace required for measurement and making measurement processing easier.

A calibration method of a fourth example of the present disclosure isfor calibrating an attitude of a camera mounted on a vehicle using aplurality of markers horizontally spaced apart in a pre-designatedpositional relationship, each marker positioned at a pre-designateddistance from a road surface. The calibration method comprises a firstprocess including shooting an image of the plurality of markers, therebygenerating a two-dimensional image; a second process includingconverting the two-dimensional image, which is generated in the firstprocess and represents the plurality of markers, into a bird's eye viewimage on a specific plane so that the bird's eye view image reflects theheight of each of the plurality of markers, wherein the specific planeis a road surface or a plane parallel to the road surface; and a thirdprocess including calculating a parameter of the camera based on adifference between a distance between the plurality of markersrepresented in the bird's eye view image on the specific plane generatedin the second process and a distance between the plurality of markersdetermined based on the pre-designated positional relationship.

According to the calibration method including the processes describedabove, the markers (targets) are positioned to have a height above theroad surface. This can save the space required for measurement comparedwith cases in which markers are placed on the road surface. Alsoaccording to the calibration method, it is necessary neither tophysically fix, using joints, the vehicle and markers to keep them in apredetermined positional relationship nor to adjust the marker positionsusing a scope means so as to bring the markers into a predeterminedpositional relationship with the vehicle. Thus, when the calibrationmethod is used, the measurement work to be performed is simpler than incases where a prior-art technique is used.

In the third process, the parameter may include at least one of avertical component of a position of the camera, a pitch angle of thecamera, and a roll angle of the camera (fifth example). Such a parametercan be accurately calculated in the third process.

Even though, the parameter value calculated in the third process may bethe value of only one of the vertical component, pitch angle and rollangle of the position of each camera, other parameter values may also becalculated as follows. Namely, at least either a yaw angle of the cameraor a horizontal component of the position of the camera is furthercalculated as the parameter of the camera based on a position differencebetween a line segment connecting the plurality of markers representedin the specific plane and a line segment connecting the plurality ofmarkers determined based on the pre-designated positional relationship(sixth example). This calculation is made based on the positiondifference between a line segment connecting a plurality of markers thatare, in the second process, represented on the road surface or plane anda line segment connecting the markers based on a pre-designatedpositional relationship.

The calibration method including the above processes makes it possibleto calculate at least either the yaw angle of the position of each ofcameras mounted on a vehicle or the horizontal components of theposition of each camera in a same coordinate system, while saving thespace required for measurement and simplifying measurement processing.

The vehicle may be mounted with either a single camera or a plurality ofcameras. When cameras are mounted, the first to the third process may beperformed for each camera (seventh example).

The calibration method including the above processes makes it possibleto calculate parameter values of each of cameras mounted on a vehicle,while saving the space required for measurement and making measurementprocessing easier.

According to an eighth example of the present disclosure, a calibrationdevice for calibrating an attitude of a camera mounted on a vehicleusing a plurality of markers each arranged vertically and eachpositioned at a pre-designated height from a road surface comprises: animage acquisition unit that acquires a two-dimensional imagerepresenting the plurality of markers shot by the camera; a conversionunit that converts the two-dimensional image acquired by the imageacquisition unit and representing the plurality of markers into a bird'seye view image on a specific plane so that the bird's eye view imagereflects the height of each of the plurality of markers, wherein thespecific plane is the road surface or on a plane parallel to the roadsurface; and a calculation unit that calculates a parameter of thecamera based on a position difference between the plurality of markersrepresented in the specific plane acquired by the conversion unit.

The parameter calculated by the calculation unit may include at leastone of a vertical component of a position of the camera, a pitch angleof the camera, and a roll angle of the camera (ninth example).

Also, the calibration device may include a plurality of cameras mountedon the vehicle and may be configured as follows (tenth example). Theimage acquisition unit acquires a plurality of two-dimensional imagesshot by each of the plurality of cameras, each two-dimensional imagerepresenting the plurality of markers. The conversion unit converts eachof the two-dimensional images acquired by the image acquisition unitinto a bird's eye view image. The calculation unit calculates aparameter of each of the plurality of camera, so that a particularmarker shot by one camera have the same position in the converted bird'seye view image as that shot by another camera, wherein the parameter ofeach camera includes at least either a yaw angle of the camera or ahorizontal component of the position of the camera.

The above calibration device can achieve effects similar to thoseachieved in the above first to third examples.

According to an eleventh example of the present disclosure, acalibration device for calibrating an attitude of a camera mounted on avehicle using a plurality of markers horizontally spaced apart in apre-designated positional relationship, each marker having apre-designated distance from a road surface comprises: an imageacquisition unit that acquires a two-dimensional image representing theplurality of markers shot by the camera; a conversion unit that convertsthe two-dimensional image acquired by the image acquisition unit andrepresenting the plurality of markers into a bird's eye view image on aspecific plane so that the bird's eye view image reflects the height ofeach of the plurality of markers, wherein the specific plane is a roadsurface or on a plane parallel to the road surface; and a calculationunit that calculates a parameter of the camera based on a differencebetween a distance between the plurality of markers represented in thebird's eye view image on the specific plane acquired by the conversionunit and a distance between the plurality of markers determined based onthe pre-designated positional relationship.

The parameter calculated by the calculation unit includes at least oneof a vertical component of a position of the camera, a pitch angle ofthe camera, and a roll angle of the camera.

Also, in the calibration device, the calculation unit (25, S140, S145)may further calculate at least either a yaw angle of the camera or ahorizontal component of the position of the camera as the parameter ofthe camera based on a position difference between a line segmentconnecting the plurality of markers represented in the specific plan anda line segment connecting the plurality of markers determined based onthe pre-designated positional relationship (thirteenth example).

Also, the calibration device may include a plurality of cameras mountedon the vehicle and may be configured such that the units perform theiroperations for each camera (fourteenth example).

The above calibration device can achieve effects similar to thoseachieved in the above fourth to seventh examples.

Exemplary embodiments and configurations according to the presentdisclosure have been described, but the embodiments and configurationsof the present disclosure are not limited to the above exemplaryembodiments and configurations. Embodiments and configurations realizedby appropriately combining technical elements disclosed in differentembodiments and configurations also fall in the range of embodiments andconfigurations according to the present disclosure.

1. A calibration method for calibrating an attitude of a camera mountedon a vehicle using a plurality of markers each arranged vertically andeach positioned at pre-designated height from a road surface, thecalibration method comprising: a first process including shooting animage of the plurality of markers with the camera, thereby generating atwo-dimensional image; a second process including converting thetwo-dimensional image, which is generated in the first process andrepresents the plurality of markers, into a bird's eye view image on aspecific plane so that the bird's eye view image reflects the height ofeach of the plurality of markers, wherein the specific plane is a roadsurface or on a plane parallel to the road surface; and a third processincluding calculating a parameter of the camera based on a positiondifference between the plurality of markers in the specific planeobtained in the second process.
 2. The calibration method according toclaim 1, wherein the parameter calculated in the third process includesat least one of a vertical component of a position of the camera, apitch angle of the camera, and a roll angle of the camera.
 3. Thecalibration method according to claim 2, wherein: the vehicle is mountedwith a plurality of the cameras; in the first process, the plurality ofmarkers are shot by each camera and the two-dimensional image isgenerated for each camera; in the second process, each of thetwo-dimensional images generated in the first process is converted intothe bird's eye view image; and in the third process, at least either ayaw angle of each camera or a horizontal component of the position ofeach camera is further calculated as the parameter of the each camera,so that a particular marker shot by one camera have the same position inthe converted bird's eye view image as the particular marker shot byanother camera.
 4. A calibration method for calibrating an attitude of acamera mounted on a vehicle using a plurality of markers horizontallyspaced apart in a pre-designated positional relationship, each markerpositioned at a pre-designated distance from a road surface, thecalibration method comprising: a first process including shooting animage of the plurality of markers, thereby generating a two-dimensionalimage; a second process including converting the two-dimensional image,which is generated in the first process and represents the plurality ofmarkers, into a bird's eye view image on a specific plane so that thebird's eye view image reflects the height of each of the plurality ofmarkers, wherein the specific plane is a road surface or a planeparallel to the road surface; and a third process including calculatinga parameter of the camera based on a difference between a distancebetween the plurality of markers represented in the bird's eye viewimage on the specific plane generated in the second process and adistance between the plurality of markers determined based on thepre-designated positional relationship.
 5. The calibration methodaccording to claim 4, wherein the parameter calculated in the thirdprocess includes at least one of a vertical component of a position ofthe camera, a pitch angle of the camera, and a roll angle of the camera.6. The calibration method according to claim 5, wherein in the thirdprocess, at least either a yaw angle of the camera or a horizontalcomponent of the position of the camera is further calculated as theparameter of the camera based on a position difference between a linesegment connecting the plurality of markers represented in the specificplane and a line segment connecting the plurality of markers determinedbased on the pre-designated positional relationship.
 7. The calibrationmethod according to claim 4, wherein: the vehicle is mounted with aplurality of the cameras; and the first process to the third process areperformed for each camera.
 8. A calibration device for calibrating anattitude of a camera mounted on a vehicle using a plurality of markerseach arranged vertically and each positioned at a pre-designated heightfrom a road surface, the calibration device comprising: an imageacquisition unit that acquires a two-dimensional image representing theplurality of markers shot by the camera; a conversion unit that convertsthe two-dimensional image acquired by the image acquisition unit andrepresenting the plurality of markers into a bird's eye view image on aspecific plane so that the bird's eye view image reflects the height ofeach of the plurality of markers, wherein the specific plane is the roadsurface or on a plane parallel to the road surface; and a calculationunit that calculates a parameter of the camera based on a positiondifference between the plurality of markers represented in the specificplane acquired by the conversion unit.
 9. The calibration deviceaccording to claim 8, wherein the parameter calculated by thecalculation unit includes at least one of a vertical component of aposition of the camera, a pitch angle of the camera, and a roll angle ofthe camera.
 10. The calibration device according to claim 9, wherein:the vehicle is mounted with a plurality of the cameras; the imageacquisition unit acquires a plurality of two-dimensional images shot byeach of the plurality of cameras, each two-dimensional imagerepresenting the plurality of markers; the conversion unit converts eachof the two-dimensional images acquired by the image acquisition unitinto a bird's eye view image; and the calculation unit calculates aparameter of each of the plurality of camera, so that a particularmarker shot by one camera have the same position in the converted bird'seye view image as that shot by another camera, wherein the parameter ofeach camera includes at least either a yaw angle of the camera or ahorizontal component of the position of the camera.
 11. A calibrationdevice for calibrating an attitude of a camera mounted on a vehicleusing a plurality of markers horizontally spaced apart in apre-designated positional relationship, each marker having apre-designated distance from a road surface, the calibration devicecomprising: an image acquisition unit that acquires a two-dimensionalimage representing the plurality of markers shot by the camera; aconversion unit that converts the two-dimensional image acquired by theimage acquisition unit and representing the plurality of markers into abird's eye view image on a specific plane so that the bird's eye viewimage reflects the height of each of the plurality of markers, whereinthe specific plane is a road surface or on a plane parallel to the roadsurface; and a calculation unit that calculates a parameter of thecamera based on a difference between a distance between the plurality ofmarkers represented in the bird's eye view image on the specific planeacquired by the conversion unit and a distance between the plurality ofmarkers determined based on the pre-designated positional relationship.12. The calibration device according to claim 11, wherein the parametercalculated by the calculation unit includes at least one of a verticalcomponent of a position of the camera, a pitch angle of the camera, anda roll angle of the camera.
 13. The calibration device according toclaim 12, wherein the calculation unit further calculates at leasteither a yaw angle of the camera or a horizontal component of theposition of the camera as the parameter of the camera based on aposition difference between a line segment connecting the plurality ofmarkers represented in the specific plan and a line segment connectingthe plurality of markers determined based on the pre-designatedpositional relationship.
 14. The calibration device according to claim11, wherein the vehicle is mounted with a plurality of the cameras, andsaid each unit performs processing for each camera.